1. Field of the Invention
This invention relates generally to a fuel cell hybrid vehicle with a propulsion system that employs an algorithm for efficiently determining a distributed power draw from a fuel cell system (FCS) and an electric energy storage system (EESS) and, more particularly, to a fuel cell hybrid vehicle that employs an algorithm for efficiently determining the distributed power draw from an FCS and an EESS by defining a power limit value and using EESS power if a power request is below the power limit value and using fuel cell system power if the power request is above the power limit value.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte there between. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
The dynamic power of a fuel cell system for a vehicle is limited. Further, the time delay from system start-up to driveability and low acceleration of the vehicle may not be acceptable. The voltage cycles can decrease the stack durability. These drawbacks can be minimized by using a high voltage battery in parallel with the fuel cell system. Algorithms are employed to provide the distribution of power from the battery and the fuel cell system to meet the requested power.
Some fuel cell vehicles are hybrid vehicles that employ an electric energy storage system (EESS) in addition to the fuel cell system, such as a DC battery or a super capacitor (also referred to as an ultra-capacitor or double layer capacitor). The EESS provides supplemental power for the various vehicle auxiliary loads, for system start-up and during high power demands when the fuel cell system is unable to provide the desired power. More particularly, the fuel cell system provides power to a traction motor and other vehicle systems through a DC voltage bus line to an electric traction system (ETS) for vehicle operation. The EESS can provide supplemental power to the voltage bus line during those times when additional power is needed beyond what the fuel cell system provides, such as during heavy acceleration. For example, the fuel cell system may provide 70 kW of power. However, vehicle acceleration may require 100 kW or more of power. The fuel cell system can be used to recharge the EESS at those times when the fuel cell system is able to meet the system power demand alone and is also able to produce additional power. The generator power available from the traction motor during regenerative braking is also used to recharge the battery through the DC bus line.
It is desirable to increase system performance, reduce hydrogen consumption, reduce component wear and tear, etc., in a fuel cell hybrid vehicle by operating the fuel cell system as efficiently as possible. Particularly, it is desirable to provide the desired mechanical output for the electric traction system by using the minimal amount of hydrogen. For a fuel cell hybrid vehicle, the hydrogen to wheel efficiency is a typical reference value that can be increased by the usage of regenerative braking and the optimized operation of the fuel cell system as the primary source within the propulsion system. To use the complete potential of regenerative braking, the EESS needs to be big enough to capture as much of the deceleration energy as possible for all power peaks. The size of the EESS is an important design consideration for the vehicle propulsion system, which is influenced and limited by cost, weight and performance requirements.
Certain operation strategies for hybrid fuel cell vehicles are primarily based on capturing the regenerative energy from vehicle braking. The control system for the hybrid power system considers this strategy by using primarily the EESS as a power source as long as the state of charge (SOC) of the EESS is within the defined thresholds. If the EESS is not able to completely provide the power request or to provide any power due to its SOC, the fuel cell system covers the power request. For high power demand requests greater than the maximum power of the fuel cell system, the EESS provides the excess power. If the SOC of the EESS does not allow for the discharge, the hybrid power system is only able to provide the maximum power from the fuel cell system.
Another part of optimizing a vehicle hybrid propulsion system is to disconnect the direct link between the power request from the vehicle based on the driver power request and the power of the primary source of the propulsion system by using the EESS as a buffer to increase the overall efficiency of the system. This optimization element needs to be integrated in the operation strategy of the propulsion system with the target to operate the fuel cell system mainly within its high efficient regions.